Vertical topology light emitting device

ABSTRACT

A vertical topology light emitting device comprises a support structure, a first adhesion layer, a second adhesion layer, a first metal layer, a second metal layer comprising a portion which directly contacts a GaN-based semiconductor structure, an interface layer, and a contact pad.

This application is a continuation of application of U.S. patentapplication Ser. No. 13/678,333 filed Nov. 15, 2012, which is acontinuation of U.S. patent application Ser. No. 12/285,557 filed Oct.8, 2008, now U.S. Pat. No. 8,106,417 issued Jan. 31, 2012, which is acontinuation of U.S. patent application Ser. No. 11/882,575 filed Aug.2, 2007, now U.S. Pat. No. 7,772,020, issued Aug. 10, 2010, which is acontinuation of U.S. patent application Ser. No. 11/497,268 filed Aug.2, 2006, now U.S. Pat. No. 8,368,115 issued Feb. 5, 2013, which is acontinuation of U.S. patent application Ser. No. 10/118,317 filed Apr.9, 2002, now U.S. Pat. No. 8,294,172 issued Oct. 23, 2012, which ishereby incorporated by reference to its entirety for all purposes as iffully incorporated herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor device fabrication. Moreparticularly, the present invention relates to a vertical device with ametal support film.

2. Discussion of the Related Art

Light emitting diodes (“LEDs”) are well-known semiconductor devices thatconvert current into light. The color of the light (wavelength) that isemitted by an LED depends on the semiconductor material that is used tofabricate the LED. This is because the wavelength of the emitted lightdepends on the semiconductor material's band-gap energy, whichrepresents the energy difference between valence band and conductionband electrons.

Gallium-Nitride (GaN) has gained much attention from LED researchers.One reason for this is that GaN can be combined with indium to produceInGaN/GaN semiconductor layers that emit green, blue, and white visiblelight. This wavelength control ability enables an LED semiconductordesigner to tailor material characteristics to achieve beneficial devicecharacteristics. For example, GaN enables an LED semiconductor designerto produce blue LEDs and blue laser diodes, which are beneficial in fullcolor displays and in optical recordings, and white LEDs, which canreplace incandescent lamps.

Because of the foregoing and other advantageous, the market forGaN-based LEDs is rapidly growing. Accordingly, GaN-basedopto-electronic device technology has rapidly evolved since theircommercial introduction in 1994. Because the efficiency of GaN lightemitting diodes has surpassed that of incandescent lighting, and is nowcomparable with that of fluorescent lighting, the market for GaN basedLEDs is expected to continue its rapid growth.

Despite the rapid development of GaN device technology, GaN devices aretoo expensive for many applications. One reason for this is the highcost of manufacturing GaN-based devices, which in turn is related to thedifficulties of growing GaN epitaxial layers and of subsequently dicingout completed GaN-based devices.

GaN-based devices are typically fabricated on sapphire substrates. Thisis because sapphire wafers are commercially available in dimensions thatare suitable for mass-producing GaN-based devices, because sapphiresupports high-quality GaN epitaxial layer growths, and because of theextensive temperature handling capability of sapphire. Typically,GaN-based devices are fabricated on 2″ diameter sapphire wafers that areeither 330 or 430 microns thick. Such a diameter enables the fabricationof thousands of individual devices, while the thickness is sufficient tosupport device fabrication without excessive wafer warping. Furthermore,the sapphire crystal is chemically and thermally stable, has a highmelting temperature that enables high temperature fabrication processes,has a high bonding energy (122.4 Kcal/mole), and a high dielectricconstant. Chemically, sapphires are crystalline aluminum oxide, Al₂O₃.

Fabricating semiconductor devices on sapphire is typically performed bygrowing an n-GaN epitaxial layer on a sapphire substrate using metaloxide chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).Then, a plurality of individual devices, such as GaN LEDs, is fabricatedon the epitaxial layer using normal semiconductor processing techniques.After the individual devices are fabricated they must be diced out ofthe sapphire substrate. However, since sapphires are extremely hard, arechemically resistant, and do not have natural cleave angles, sapphiresubstrates are difficult to dice. Indeed, dicing typically requires thatthe sapphire substrate be thinned to about 100 microns by mechanicalgrinding, lapping, and/or polishing. It should be noted that suchmechanical steps are time consuming and expensive, and that such stepsreduce device yields. Even after thinning sapphires remain difficult todice. Thus, after thinning and polishing, the sapphire substrate isusually attached to a supporting tape. Then, a diamond saw or stylusforms scribe lines between the individual devices. Such scribingtypically requires at least half an hour to process one substrate,adding even more to the manufacturing costs. Additionally, since thescribe lines have to be relatively wide to enable subsequent dicing, thedevice yields are reduced, adding even more to manufacturing costs.After scribing, the sapphire substrates are rolled using a rubber rollerto produce stress cracks that propagate from the scribe lines and thatsubsequently dice out the individual semiconductor devices. Thismechanical handling reduces yields even more.

In addition to the foregoing problem of dicing individual devices fromsapphire substrates, or in general other insulating substrate, sapphiresubstrates or other insulating substrate have other drawbacks. Of note,because sapphire is an insulator, the device topologies that areavailable when using sapphire substrates (or other insulatingsubstrates) are limited. In practice there are only two devicetopologies: lateral and vertical. In the lateral topology the metallicelectrical contacts that are used to inject current are both located onupper surfaces. In the vertical topology the substrate is removed, onemetallic contact is on the upper surface and the other contact is on thelower surface.

FIGS. 1A and 1B illustrate a typical lateral GaN-based LED 20 that isfabricated on a sapphire substrate 22. Referring now specifically toFIG. 1A, an n-GaN buffer layer 24 is formed on the substrate 22. Arelatively thick n-GaN layer 26 is formed on the buffer layer 24. Anactive layer 28 having multiple quantum wells ofaluminum-indium-gallium-nitride (AlInGaN) or of InGaN/GaN is then formedon the n-type GaN layer 26. A p-GaN layer 30 is then formed on theactive layer 26. A transparent conductive layer 32 is then formed on thep-GaN layer 30. The transparent conductive layer 32 may be made of anysuitable material, such as Ru/Au, Ni/Au or indium-tin-oxide (ITO). Ap-type electrode 34 is then formed on one side of the transparentconductive layer 32. Suitable p-type electrode materials include Ni/Au,Pd/Au, Pd/Ni and Pt. A pad 36 is then formed on the p-type electrode 34.Beneficially, the pad 36 is Au. The transparent conductive layer 32, thep-GaN layer 30, the active layer 28 and part of the n-GaN layer 26 areetched to form a step. Because of the difficulty of wet etching GaN, adry etch is usually used to form the step. This etching requiresadditional lithography and stripping processes. Furthermore, plasmadamage to the GaN step surface is often sustained during the dry-etchprocess. The LED 20 is completed by forming an n-electrode pad 38(usually Au) and pad 40 on the step.

FIG. 1B illustrates a top down view of the LED 20. As can be seen,lateral GaN-based LEDs have a significant draw back in that having bothmetal contacts (36 and 40) on the same side of the LED significantlyreduces the surface area available for light emission. As shown in FIG.1B the metal contacts 36 and 40 are physically close together.Furthermore, as previously mentioned the pads 36 are often Au. Whenexternal wire bonds are attached to the pads 36 and 40 the Au oftenspreads. Au spreading can bring the electrical contacts even closertogether. Such closely spaced electrodes 34 are highly susceptible toESD problems.

FIGS. 2A and 2B illustrate a vertical GaN-based LED 50 that was formedon a sapphire substrate that was later removed. Referring nowspecifically to FIG. 2A, the LED 50 includes a GaN buffer layer 54having an n-metal contact 56 on a bottom side and a relatively thickn-GaN layer 58 on the other. The n-metal contact 56 is beneficiallyformed from a high reflectively layer that is overlaid by a highconductivity metal (beneficially Au). An active layer 60 having multiplequantum wells is formed on the n-type GaN layer 58, and a p-GaN layer 62is formed on the active layer 60. A transparent conductive layer 64 isthen formed on the p-GaN layer 62, and a p-type electrode 66 is formedon the transparent conductive layer 64. A pad 68 is formed on the p-typeelectrode 66. The materials for the various layers are similar to thoseused in the lateral LED 20. The vertical GaN-based LED 50 as theadvantage that etching a step is not required. However, to locate then-metal contact 56 below the GaN buffer layer 54 the sapphire substrate(not shown) has to be removed. Such removal can be difficult,particularly if device yields are of concern. However, as discussedsubsequently, sapphire substrate removal using laser lift off is known.(see, U.S. Pat. No. 6,071,795 to Cheung et al., entitled, “Separation ofThin Films From Transparent Substrates By Selective Optical Processing,”issued on Jun. 6, 2000, and Kelly et al. “Optical process for liftoff ofgroup III-nitride films”, Physica Status Solidi (a) vol. 159, 1997, pp.R3-R4).

Referring now to FIG. 2B, vertical GaN-based LEDs have the advantagethat only one metal contact (68) blocks light emission. Thus, to providethe same amount of light emission area lateral GaN-based LEDs must havelarger surface areas, which causes lower device yields. Furthermore, thereflecting layer of the n-type contact 56 used in vertical GaN-basedLEDs reflect light that is otherwise absorbed in lateral GaN-based LEDs.Thus, to emit the same amount of light as a vertical GaN-based LED, alateral GaN-based LED must have a significantly larger surface area.Because of these issues, a 2″ diameter sapphire wafer can produce about35,000 vertical GaN-based LEDs, but only about 12,000 lateral GaN-basedLEDs. Furthermore, the lateral topology is more vulnerable to staticelectricity, primarily because the two electrodes (36 and 40) are soclose together. Additionally, as the lateral topology is fabricated onan insulating substrate, and as the vertical topology can be attached toa heat sink, the lateral topology has relatively poor thermaldissipation. Thus, in many respects the vertical topology isoperationally superior to the lateral topology.

However, most GaN-based LEDs fabricated on insulating substrates have alateral topology. This is primarily because of the difficulties ofremoving the insulating substrate and of handling the GaN waferstructure without a supporting substrate. Despite these problems,removal of an insulating (growth) substrate and subsequent wafer bondingof the resulting GaN-based wafer on a Si substrate using Pd/In metallayers has been demonstrated for very small area wafers, approx. 1 cm by1 cm. (reported by the University of California at Berkley and the XeroxCorporation). But, substrate removal and subsequent wafer bonding oflarge area wafers remains very difficult due to inhomogeneous bondingbetween the GaN wafer and the 2^(nd) (substitutional) substrate. This ismainly due to wafer bowing during and after laser lift off.

Thus, it is apparent that a better method of substituting a 2^(nd)(substitutional) substrate for the original (growth) insulatingsubstrate would be beneficial. In particular, a method that provides formechanical stability of the wafer, that supports good electricalcontact, and that assists heat dissipation would be highly useful,particularly for devices subject to high electrical current injection,such as laser diodes or high power LEDs. This would enable formingsemiconductor layers on an insulating substrate, followed by removal ofthe insulating substrate to isolate a wafer having the formedsemiconductor layers, followed by subsequent attachment of the wafer toa metal substitutional substrate. Of particular benefit would be a newmethod suitable for removing sapphire substrates from partiallyfabricated semiconductor devices, particularly if those devices areGaN-based. For example, a method of removing semiconductor layers from asapphire substrate, of isolating a wafer having the partially fabricatedsemiconductor devices such that wafer warping is reduced or prevented,followed by substitution of a metal supporting layer would be useful.More specifically, a method of partially fabricating GaN-based deviceson a sapphire (or other insulating) substrate, followed by substitutionof a conducting supporting layer, followed by dicing the substitutinglayer to yield vertical topology GaN-based LEDs would be beneficial.

SUMMARY OF THE INVENTION

The following summary of the invention is provided to facilitate anunderstanding of some of the innovative features unique to the presentinvention, and is not intended to be a full description. A fullappreciation of the various aspects of the invention can be gained bytaking the entire specification, claims, drawings, and abstract as awhole

The principles of the present invention provide for a method offabricating semiconductor devices on insulating substrates by firstforming semiconductor layers on the insulating substrate, followed byremoval of the insulating substrate to isolate a wafer having the formedsemiconductor layers, followed by the addition of a metal supportsubstrate (either on top or bottom of semiconductor layers) that willsupport the wafer, all while supporting the wafer to prevent warpingand/or other damage.

The principles of the present invention further provide for a method offabricating GaN-based vertical devices on insulating substrates usingmetal support films. According to that method, semiconductor layers forthe GaN-based devices are formed on an insulating (sapphire) substrateusing normal semiconductor fabrication techniques. Then, trenches areformed through the semiconductor layers and into the insulatingsubstrate. Beneficially, the trenches are fabricated using inductivecouple (inductively coupled) plasma reactive ion etching (ICPRIE). Then,a first support structure is attached to the semiconductor layers.Beneficially, the first support structure is comprised of silicon, butalmost any hard flat surface is acceptable. That first support structureis beneficially attached to the semiconductive layers using an epoxyadhesive, possibly with a protective photo-resist layer over thesemiconductive layer. Then, the insulating substrate is removed,beneficially using a laser-lift off process. A second supportingstructure is then substituted for the insulating substrate.Beneficially, the second supporting structure is comprised of a metalfilm of Cu, Au or Al, but almost any conductive film is acceptable. Ifrequired, a conductive contact can be inserted between thesemiconductive layer and the second supporting structure. In the case ofLEDs, the conductive contact is beneficially reflective to bouncephotons upward to prevent absorption in the bottom lead frame. The firstsupporting structure is then removed. Individual devices are then dicedout, beneficially either by mechanical dicing or wet/dry etching throughthe second supporting structure.

The following describes another way of forming metal support films onthe semiconductor layers. Trench formation through the semiconductorlayers and into the insulating substrate is identical to the proceduredescribed above. Then, instead of attaching the semiconductor layersonto the support structure (Si or a hard flat surface), a thick metalsupport film is deposited on top of the GaN-based devices using chemicaland/or physical deposition techniques (such as electroplating orelectro-less plating). Then, the insulating substrate is removed,beneficially using a laser-lift off process. Beneficially, the thickmetal support film is comprised of Cu, Au or Al, but almost anyconductive film is acceptable. If required, a conductive contact can beinserted between the semiconductive layer and the second supportingstructure. In the case of LEDs, the conductive contact is beneficiallyreflective to bounce photons to prevent absorption in the bottom leadframe. Electrical contacts can then be formed on the exposed surface ofthe semiconductor layers. Individual devices can then diced out,beneficially either by mechanical dicing or wet/dry etching through thethick metal support film.

The principles of the present invention specifically provide for amethod of fabricating vertical topology GaN-based LEDs on sapphiresubstrates. According to that method, semiconductor layers for thevertical topology GaN-based LEDs are formed on a sapphire substrateusing normal semiconductor fabrication techniques. Then, trenches areformed through the semiconductor layers and into the sapphire substrate.Those trenches define the boundaries of the individual vertical topologyGaN-based LEDs. Beneficially, the trenches are fabricated using ICPRIE.Then, a protective photo-resist layer is located over the semiconductorlayers. A first support structure is then attached to the semiconductorlayers. Beneficially, the first support structure is a silicon plate,but almost any hard flat material is acceptable. The first supportstructure is beneficially attached to the semiconductive layers (orphoto-resist layer) using an epoxy adhesive. Then, the sapphiresubstrate is removed, beneficially using a laser lift off process. Aconductive bottom contact is then located on the exposed semiconductorlayer. That conductive bottom contact beneficially includes a reflectivelayer. One or more adhesion support layers, such as a Cr and/or and Aulayer, is formed over the reflective layer. Then, a second supportingstructure is substituted in place of the sapphire substrate.Beneficially, the second supporting structure is comprised of aconductive film of Cu, Au or Al, but almost any conductive film isacceptable. The first supporting structure is then removed. Finally, theindividual device dies are diced out, beneficially either by mechanicaldicing or by wet/dry etching through the second supporting structure.Mechanical rolling or shear cutting can be used to separate the dies.

The principles of the present invention also provide for another methodof fabricating vertical topology GaN-based LEDs on sapphire substrates.According to that method, semiconductor layers for the vertical topologyGaN-based LEDs are formed on a sapphire substrate using normalsemiconductor fabrication techniques. Then, trenches are formed throughthe semiconductor layers and into the sapphire substrate. Those trenchesdefine the boundaries of the individual vertical topology GaN-basedLEDs. Beneficially, the trenches are fabricated using ICPRIE. Then, acontact layer comprised, for example, of layers of Cr and Au is locatedover the semiconductor layers. Then a metal support structure is thenformed over the contact layer/semiconductor layers. Then, the sapphiresubstrate is removed, beneficially using a laser lift off process.Conductive bottom contacts are then located on the recently exposedsemiconductor layer. Finally, the individual device dies are diced out,beneficially either by mechanical dicing or by wet/dry etching throughthe metal support structure.

The novel features of the present invention will become apparent tothose of skill in the art upon examination of the following detaileddescription of the invention or can be learned by practice of thepresent invention. It should be understood, however, that the detaileddescription of the invention and the specific examples presented, whileindicating certain embodiments of the present invention, are providedfor illustration purposes only because various changes and modificationswithin the spirit and scope of the invention will become apparent tothose of skill in the art from the detailed description of the inventionand claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form part of the specification,further illustrate the present invention and, together with the detaileddescription of the invention, serve to explain the principles of thepresent invention.

In the drawings:

FIG. 1A illustrates a sectional view of a typical lateral topologyGaN-based LED;

FIG. 1B shows a top down view of the GaN-based LED illustrated in FIG.1A;

FIG. 2A illustrates a sectional view of a typical vertical topologyGaN-based LED;

FIG. 2B shows a top down view of the GaN-based LED illustrated in FIG.2A; and

FIGS. 3-25 illustrate steps of forming light emitting diodes that are inaccord with the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the present invention provide for methods offabricating GaN-based vertical devices on insulating substrates usingthick metal support films. While those principles are illustrated in adetailed description of a method of fabricating vertical topologyGaN-based LEDs on a sapphire substrate, those principles are broaderthan that method. Therefore, the principles of the present invention areto be limited only by the appended claims as understood under UnitedStates Patent Laws.

FIGS. 3-25 illustrate methods of manufacturing vertical topologyGaN-based light emitting diodes (LEDs) using sapphire substrates.Sapphire substrates are readily available in suitable sizes, arethermally, chemically, and mechanically stable, are relativelyinexpensive, and support the growth of good quality GaN epitaxiallayers.

Referring now to FIG. 3, a vertical topology GaN-based LED layerstructure 120 that is similar or identical to the semiconductor layersof the vertical GaN-based LED 50 illustrated in FIGS. 2A and 2B isformed on a 330-430 micron-thick, 2″ diameter sapphire substrate 122.For example, the vertical topology GaN-based LED layer structure 120 canhave an InGaN/GaN active layer (60) having the proper composition toemit blue light. The vertical topology GaN-based LED layer structure 120is beneficially less than 5 microns thick. Various standard epitaxialgrowth techniques, such as vapor phase epitaxy, MOCVD, and MBE, togetherwith suitable dopants and other materials, can be used to produce thevertical topology GaN-based LED layer structure 120.

Referring now to FIG. 4, trenches 124 are formed through the verticaltopology GaN-based LED layer structure 120 and into the sapphiresubstrate 122. The trenches define the individual LED semiconductorstructures that will be produced and separated. Each individual LEDsemiconductor structure is beneficially a square about 200 microns wide.The trenches are beneficially narrower than about 10 microns wide andextend deeper than about 5 microns into the sapphire substrate 122.

Because of the hardness of sapphire and GaN, the trenches 124 arebeneficially formed in the structure of FIG. 3 using reactive ionetching, preferably inductively coupled plasma reactive ion etching (ICPRIE). Forming trenches using ICP RIE has two main steps: forming scribelines and etching. Scribe lines are formed on the structure of FIG. 3using a photo-resist pattern in which areas of the sapphire substrate122 where the trenches 124 are to be formed are exposed. The exposedareas are the scribe lines and all other areas are covered byphoto-resist. The photo-resist pattern is beneficially fabricated from arelatively hard photo-resist material that withstands intense plasma.For example, the photo-resist could be AZ 9260, while the developer usedto develop the photo-resist to form the scribe lines could be AZ MIF500.

In the illustrated example, the photo-resist is beneficially spin coatedto a thickness of about 10 microns. However, in general, thephoto-resist thickness should be about the same as the thickness of thevertical topology GaN-based LED layer structure 120 plus the etch depthinto the sapphire substrate 122. This helps ensure that the photo-resistmask remains intact during etching. Because it is difficult to form athick photo-resist coating in one step, the photo-resist is beneficiallyapplied in two coats, each about 5 microns thick. The first photo-resistcoat is spin coated on and then soft baked at approximately 90° F. forabout 15 minutes. Then, the second photo-resist coat is applied in asimilar manner, but is soft baked at approximately 110° F. for about 8minutes. The photo-resist coating is then patterned to form the scribelines. This is beneficially performed using lithographic techniques anddevelopment. Development takes a relatively long time because of thethickness of the photo-resist coating. After development, thephoto-resist pattern is hard baked at about 80° F. for about 30 minutes.Then, the hard baked photo-resist is beneficially dipped in a MCB (MetalChlorobenzene) treatment for about 3.5 minutes. Such dipping furtherhardens the photo-resist.

After the scribe lines are defined, the structure of FIG. 3 is etched.Referring now to FIG. 5, the ICP RIE etch process is performed byplacing the structure of FIG. 3 on a bottom electrode 350 in a RIEchamber 352 having an insulating window 354 (beneficially a 1 cm-thickquartz window). The bottom electrode 350 is connected to a bias voltagesupply 356 that biases the structure of FIG. 3 to enable etching. Thebias voltage supply 356 beneficially supplies 13.56 MHz RF power and aDC-bias voltage. The distance from the insulating window 354 to thebottom electrode 350 is beneficially about 6.5 cm. A gas mixture of Cl₂and BCl₃, and possibly Ar, is injected into the RIE chamber 352 througha reactive gas port 360. Furthermore, electrons are injected into thechamber via a port 362. A 2.5-turn or so spiral Cu coil 364 is locatedabove the insulating window 354. Radio frequency (RF) power at 13.56 MHzis applied to the coil 364 from an RF source 366. It should be notedthat magnetic fields are produced at right angles to the insulatingwindow 354 by the RF power.

Still referring to FIG. 5, electrons present in the electromagneticfield produced by the coil 364 collide with neutral particles of theinjected gases, resulting in the formation of ions and neutrals, whichproduce plasma. Ions in the plasma are accelerated toward the structureof FIG. 3 by the bias voltage applied by the bias voltage supply 356 tothe bottom electrode 350. The accelerated ions pass through the scribelines, forming the etch channels 124 (see FIG. 4).

With the structure of FIG. 4, fabrication proceeds using one of twogeneral procedures. The first procedure is to form a temporary substrateon top of the structure of FIG. 4. The other is to form a permanentmetal layer on top of the structure of FIG. 4. The formation of atemporary substrate will be described first (with reference to FIGS. 6through 15), followed by a description of the use of a permanent metallayer (with reference to FIGS. 16-20).

Referring now to FIG. 6, after the trenches 124 are formed, thintransparent contacts 190 are formed on the individual LED semiconductorstructures of the vertical topology GaN-based LED layer structure 120.Those transparent contacts 190 are beneficially comprised of Ru/Au,Ni/Au, or of indium tin oxide (ITO)/Au and are less than 10 nm. As shownin FIG. 7, after the transparent contacts 190 are formed, metal contactpads 192 are placed on each transparent contact 190. The metal contactpads 192 are beneficially comprised of Pd, Pt, Au, or Al. Each metalcontact pad 192 has a diameter of about 100 microns and a thickness ofabout 1 micron. A thin Cr/Au inter layer can be used to improve adhesionbetween transparent contacts 190 and the metal contact pad 192.

Referring now to FIG. 8, a protective photo-resist film 196 is formedover the structure of FIG. 7. That photo-resist film is to protect theGaN-based LED layer structure 120 and to assist subsequent bonding. Anepoxy adhesive 198 is then used to attach a first supporting structurethat takes the form of a temporary supporting wafer 200. The temporarysupporting wafer 200 is beneficially a silicon plate that is larger thanthe sapphire wafer. However, almost any hard, flat surface with asufficient thickness to support a wafer containing the individual LEDsemiconductor devices during substrate swapping (described subsequently)is acceptable. Still referring to FIG. 8, the first substrate swappingprocesses is surface polishing and sand blasting (or surface rougheningwith a dry etching processes) the backside (the bottom side in FIG. 8)of the sapphire substrate 122. This step helps to ensure uniform laserbeam heating during a laser lift off step that is subsequentlyperformed.

Turning now to FIG. 9, the structure shown in FIG. 8 is then attached totwo vacuum chucks. A first vacuum chuck 210 attaches to the supportingwafer 200 and the second vacuum chuck 212 attaches to the sapphiresubstrate 122. Then, still with reference to FIG. 9, a laser beam 214 isdirected through the sapphire substrate 122. The laser beam 214 isbeneficially from a 248 nm KrF laser having a 3 mm×50 mm rectangularbeam and beam energy between 200˜600 mJ/cm². The vacuum chucks 210 and212, which are made of materials transparent to the 248 nm KrF laserbeam, beneficially sapphire, bias the sapphire substrate 122 away fromthe supporting wafer 200. The combination of laser irradiation and biascauses the sapphire substrate 122 to separate as shown in FIG. 10.

Similar laser lift off processes are described in U.S. Pat. No.6,071,795 to Cheung et al., entitled, “Separation of Thin Films FromTransparent Substrates By Selective Optical Processing,” issued on Jun.6, 2000, and in Kelly et al. “Optical process for liftoff of groupIII-nitride films,” Physica Status Solidi (a) vol. 159, 1997, pp. R3-R4.Beneficially, the temporary supporting wafer 200 fully supports theindividual LED semiconductor structures in the vertical topologyGaN-based LED layer structure 120 in a manner the resists warping.

Turning now to FIG. 11, after the sapphire substrate 122 is removed, thebottom of the resulting structure (the side opposite the temporarysupporting wafer 200) is first cleaned with HCl to remove Ga droplets(the laser beam 214 causes heating which separates the GaN into Ga+N).After cleaning, ICP RIE etching (see above) and polishing are performed.This etching and polishing exposes and produces an atomically flatsurface of pure n-GaN. The flat surface is particularly beneficial inproducing high reflectivity from a reflective structure that isdeposited subsequently. Prior to reflective layer deposition, the etchedn-GaN surface is further cleaned and etched with aqua regia solution(mixture of H₂SO₄ and HCl) to enhance the adhesion between n-GaN andTi/Al metal layers.

Turning now to FIG. 12, a conductive reflective structure comprised of atitanium layer 230 and an aluminum layer 232 is then formed on thebottom of the structure of FIG. 11. That reflective structure willreflect light from completed LEDs that is directed toward the bottom ofthe LEDs back out of the top of the LEDs. These bottom metal layers alsoserve as an n-type contact layer for the LED device.

Turning now to FIG. 13, to assist formation of a subsequently producedsecond supporting structure, a Cr adhesion layer 236, which is less thanabout 30 nm thick, is formed on the Al layer 232 and an Au adhesionlayer 238, which is less than about 100 nm thick, is formed on the Cradhesion layer 236.

Turning now to FIG. 14, after the Au adhesion layer 238 is in place asecond supporting structure in the form of a Cu, Au or Al thick filmsupport 240 is formed on the Au adhesion layer 238. The thick filmsupport 240 can be formed by physical vapor deposition byelectroplating, by electro-less plating, or by other suitable means.This thick film support 240 is beneficially less than about 100 micronsthick. While a Cu, Au or Al thick film support is beneficial, almost anyelectrically conductive, and beneficially thermally conductive, materialis acceptable.

After the thick support 240 is in place, the epoxy adhesive 198 and thetemporary supporting wafer 200 are removed, reference FIG. 15. Suchremoval is beneficially achieved by heating the structure of FIG. 14 toweaken the epoxy adhesive such that the temporary supporting wafer 200can be removed. After the temporary supporting wafer 200 is removed theresulting structure is immersed in acetone to remove any photo-resistand residual epoxy adhesive 198.

The process steps illustrated in FIGS. 6 through 15 provide for ageneral fabrication process that uses a temporary support structure 200.Referring now to FIG. 16, an alternative method uses a thick metalsupport film 300 that is formed on top of the structure of FIG. 4.

First, a transparent metal layer 290 is formed on the vertical topologyGaN-based LED layer structures 120. Then, an adhesion layer 338comprised of Cr and Au layers is located on the transparent metal layer290. Then, the thick metal support film 300, beneficially comprised ofCu, Au or Al, is formed on the adhesion layer 338. The thick metalsupport film 300 can be formed by physical vapor deposition,electro/electro-less plating, or by other suitable means. This thickmetal support film 300 is beneficially less than about 100 micronsthick. While a Cu, Au or Al thick metal support film 300 is beneficial,almost any electrically conductive, and beneficially thermallyconductive, material is acceptable.

Turning now to FIG. 17, the structure shown in FIG. 16 is then attachedto two vacuum chucks. A first vacuum chuck 210 attaches to the thickmetal support film 300 and the second vacuum chuck 212 attaches to thesapphire substrate 122. Then, still with reference to FIG. 17, a laserbeam 214 is directed through the sapphire substrate 122. The laser beam214 is beneficially from a 248 nm KrF laser with 3 mm×50 mm rectangularbeam and beam energy in between 200˜600 mJ/cm2. The vacuum chucks 210and 212, which are made of materials transparent to the 248 nm KrF laserbeam, beneficially sapphire, bias the sapphire substrate 122 away fromthe GaN-LED devices backed with thick metal support film 300. Thecombination of laser irradiation and bias causes the sapphire substrate122 to separate as shown in FIG. 18.

Similar laser lift off processes are described in U.S. Pat. No.6,071,795 to Cheung et al., entitled, “Separation of Thin Films FromTransparent Substrates By Selective Optical Processing,” issued on Jun.6, 2000, and in Kelly et al. “Optical process for liftoff of groupIII-nitride films,” Physica Status Solidi (a) vol. 159, 1997, pp. R3-R4.Beneficially, the supporting wafer 200 fully supports the individual LEDsemiconductor structures in the vertical topology GaN-based LED layerstructure 120.

Turning now to FIG. 19, after the sapphire substrate 122 is removed, thebottom of the resulting structure (the side opposite the thick metalfilm 240) is first cleaned with HCl to remove Ga droplets (the laserbeam 214 causes heating which separates the GaN into Ga+N). Aftercleaning, ICP RIE etching (see above) and polishing are performed. Thisetching and polishing exposes and produces an atomically flat surface ofpure n-GaN. Prior to n-type contact formation, the etched n-GaN surfaceis further cleaned and etched with aqua regia solution (mixture of H₂SO₄and HCl) to enhance the adhesion between n-GaN and Ti/Al metal layers.

Referring now to FIG. 20, after etching and polishing exposes andproduces an atomically flat surface (see FIG. 19), electrical contactsare formed on the individual vertical topology GaN-based LED layerstructures 120. Those electrical contacts beneficially include a Ti/Alinterface layer 330 to the vertical topology GaN-based LED layerstructures 120, and a Cr/Au contact pad 332 on the Ti/Al interface layer330.

After removal of the temporary supporting wafer 200 to leave thestructure shown in FIG. 15, or after formation of the Cr/Au contactlayer 332 to leave the structure shown in FIG. 20, the individual LEDdevices are ready to be diced out. Dicing can be accomplished in manyways, for example, by chemical/electrochemical etching or by mechanicalaction. As the basic dicing operations are the same, dicing will bedescribed with specific reference to the structure shown in FIG. 15,with the understanding that dicing the structure of FIG. 20 is similar.Referring now to FIG. 21, dicing is beneficially accomplished bydepositing a photo-resist pattern 250 on the thick film support 240.That photo-resist pattern 250 is then developed to expose areas of thethick film support 240 that align with the trenches 124. Openings 254are then etched through the thick film support 240. The photo-resistpattern 250 is then removed.

Actual separation of the individual devices can be accomplished inseveral ways. For example, as shown in FIG. 22, a mounting tape 260 canbe placed on top of the structure of FIG. 21. Then, a roller can rollover the mounting tape to stress the remaining intact layers such thatthe individual devices are diced out. Alternatively, as shown in FIG.23, the mounting tape 260 can be located on the bottom of the structureof FIG. 21. Then, a diamond-cutting wheel 262 can dice out theindividual devices.

The result is a plurality of vertical topology GaN LEDs 199 onconductive substrates. As shown in FIG. 24, each LED includes a thickfilm support 240, an adhesion support (Cr adhesion layer 236 and Auadhesion layer 238), a reflective structure (titanium layer 230 andaluminum layer 232), semiconductor layers 120 and top contacts(transparent contact 190 and metal contact pad 192). Those semiconductorlayers include semiconductor layers as shown in FIG. 2A.

Alternatively, if a thick metal support film 300 is used, the result isthe LED 399 shown in FIG. 25. That LED includes a thick metal supportfilm 300, an adhesion layer 338, a reflective and p-type transparentcontact 290, semiconductor layers 120, an n-type top interface layer330, and a contact pad 332. Those semiconductor layers includesemiconductor layers as shown in FIG. 2A.

The embodiments and examples set forth herein are presented to bestexplain the present invention and its practical application and tothereby enable those skilled in the art to make and utilize theinvention. Those skilled in the art, however, will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. Other variations and modifications ofthe present invention will be apparent to those of skill in the art, andit is the intent of the appended claims that such variations andmodifications be covered. The description as set forth is not intendedto be exhaustive or to limit the scope of the invention. Manymodifications and variations are possible in light of the above teachingwithout departing from the spirit and scope of the following claims. Itis contemplated that the use of the present invention can involvecomponents having different characteristics. It is intended that thescope of the present invention be defined by the claims appended hereto,giving full cognizance to equivalents in all respects.

What is claimed is:
 1. A light emitting device, comprising: a supportstructure; a first adhesion layer disposed on the support structure, thefirst adhesion layer comprising Au; a second adhesion layer disposed onthe first adhesion layer; a first metal layer disposed on the secondadhesion layer; a second metal layer disposed on the first metal layer,the second metal layer comprising Ti; a GaN-based semiconductorstructure disposed on the second metal layer; an interface layerdisposed on the GaN-based semiconductor structure; and a contact paddisposed on the interface layer, wherein the second metal layercomprises a portion which directly contacts the GaN-based semiconductorstructure.
 2. The device according to claim 1, wherein the supportstructure comprises Cu.
 3. The device according to claim 1, wherein thefirst adhesion layer directly contacts the second adhesion layer.
 4. Thedevice according to claim 1, wherein the first adhesion layer is thickerthan the second adhesion layer.
 5. The device according to claim 1,wherein the first metal layer directly contacts the second metal layer.6. The device according to claim 1, wherein the first metal layer isthicker than the second metal layer.
 7. The device according to claim 1,wherein the GaN-based semiconductor structure comprises an active layerhaving multiple quantum well structure, and wherein the active layercomprises AlInGaN.
 8. The device according to claim 1, wherein theGaN-based semiconductor structure comprises an etched n-type GaN.
 9. Thedevice according to claim 1, wherein the GaN-based semiconductorstructure is less than 5 microns thick.
 10. The device according toclaim 1, wherein the interface layer comprises Ti.
 11. The deviceaccording to claim 1, wherein the interface layer comprises a firstinterface layer and a second interface layer.
 12. The device accordingto claim 1, wherein the contact pad has a diameter of about 100 microns.13. The device according to claim 1, further comprising a metal contactwhich contacts a p-type layer of the GaN-based semiconductor structure.14. The device according to claim 1, wherein the portion of the secondmetal layer directly contacts a p-type layer of the GaN-basedsemiconductor structure.
 15. A light emitting device, comprising: asupport structure; an adhesion structure disposed on the supportstructure; a first metal layer disposed on the adhesion structure; asecond metal layer disposed on the first metal layer, the second metallayer comprising Ti; and a GaN-based semiconductor structure disposed onthe adhesion structure, wherein the second metal layer comprises aportion which directly contacts the GaN-based semiconductor structure,and wherein the second metal layer serves as an n-type electrode to theGaN-based semiconductor structure.
 16. The device according to claim 15,wherein the support structure comprises Cu.
 17. The device according toclaim 15, wherein the adhesion structure comprises Au.
 18. The deviceaccording to claim 15, wherein the adhesion structure comprises a firstadhesion layer and a second adhesion layer.
 19. The device according toclaim 15, wherein the first metal layer comprises Al.
 20. The deviceaccording to claim 15, further comprising a metal contact which contactsa p-type layer of the GaN-based semiconductor structure.